Biomaterials - Principles of Biomedical Engineering - page 226

two to three times less than the wear rate of UHMWPE against stainless steel. The
wear rate of UHMWPE against alumina ceramic is about 20 times lower compared
to polyethylene against Co-Cr-Mo alloy. However, alumina ceramic exhibits a brit-
tle tendency and is sensitive to microstructural flaws.
There are a number of endogenous factors and exogenous factors that affect
the mechanical durability. Endogenous factors include the starting raw material,
the manufacturing process, the thickness of the device, the rupture energy of the
material, sterilization methods, packaging conditions, and aging. For example, the
wear resistance of UHMWPE is affected by sterilization techniques. Gamma irra-
diation in air breaks molecular bonds in the UHMWPE chains, giving rise to free
radicals. Oxygen present in the environment combines with these free radicals,
leading to subsurface oxidation. An increase in oxidation increases fatigue cracking
and delamination. Components that have been on the shelf for less than a year be-
fore implantation show decreased in vivo oxidation and better in vivo performance
than those with longer shelf lives. Laboratory wear studies have shown increased
wear rates in polyethylene gamma irradiated in the air compared to nonirradiated
material. Endogenous factors include design of bearing components, level of con-
formity, alignment, surface conditioning, and modularity. For example, the wear
rate of certain polymer materials has been demonstrated to be highly dependent
on the motion-path or crossings, which occur during the relative motion of the
impinging surfaces. The wear rates of hip implants using UHMWPE are related to
the crossings of the wear path.
6.3.3 Corrosion and Degradation
Corrosion and degradation are two processes that determine the nature and quan-
tity of wear debris and ions produced during the lifetime of implants. Corrosion oc-
curs due to the interaction between a solid material and its chemical environment,
which leads to a loss of substance from the material, a change in its structural char-
acteristics, or a loss of structural integrity. These processes take place at the surface
of the implant and the surface of wear debris produced by physical processes. The
resistance of a device to chemical or structural degradation (biostability), and the
nature of the reactions that occur at the biological interface determine the corro-
sion characteristics of biomaterials. Ceramics are extremely inert to corrosion. The
implant material is only available for chemical reaction at the surface.
Corrosion of metals is widely investigated and there are many types of corro-
sion. Pitting corrosion arises from a breakdown of protective film. Frequently the
pitting is initiated at inclusions, indicating the importance of clean metals. Fretting
corrosion typically arises from micromotions occurring at the tissue/implant inter-
face in addition to the action of mechanical loading and electrochemical oxidation.
The main mechanical factors are the contact pressure, slip amplitude at the inter-
face, and the frequency of movement, the latter depending on the age and activity
of the patient. Osteoporosis affects the mechanical properties of bone and thus in-
fluences the stiffness of the contact. As a result, oxide particles accumulate and act
as abrasive particles between the rubbing surfaces. This leads to the degradation of
the implant and loosening of the prosthesis.